A vehicle having two axles which can be directly coupled to a power source such as an internal combustion engine can stop with a relatively short braking distance as long as its tires ware within their slip limits, but, since the two axles are rigidly coupled to each other, no matter how ideally braking forces are applied to each of the tires, the braking forces are commonly transmitted to both the axles through the path of power transmission and rational distribution of braking forces to the different axles is not possible by any ordinary means.
In a four wheel drive vehicle using a viscous coupling in which torque is transmitted between the front and the rear axle according to the difference in the rotational speed of the two axles, this problem is much improved as compared to a simple four wheel drive vehicle in which the front and the rear axle are directly coupled by way of gears or the like which do not permit any slippage between the two axles, but this solution is based on a compromise or a trade-off. If the viscous coupling is hard, the advantage of four wheeel drive is enhanced but a favorble distribution of braking forces between the front and the rear axle cannot be accomplished. On the other hand, if the viscous coupling is soft, the advantage of the four wheel drive is lost but an unfavorable distribution of the braking forces can be avoided.
In view of this problem, it has been proposed in Japanese Patent Laid Open Publication No. 59-188731 to selectively disconnect the power transmission between the front axle or the engine and the rear axle by means of a one-way clutch. Thus, a drive torque can be transmitted from the front axle to the rear axle, but the braking force of the front axle is prevented from being transmitted to the rear axlee. However, according to this proposal, the braking action of the engine is also prevented from being transmitted to the rear axle. This is not desirable because engine brake is essential for a favorable handling of a vehicle powered by an internal combustion engine.
To the end of eliminating the above mentioned problems of four wheel drive vehicles using viscous couplings and other couplings whose power transmission properties depend on the difference in rotational speed between an input end and an output end thereof, it was proposed in a copending U.S. patent application No. 148,781 (based on Japanese patent application No. 62-17733 filed on Jan. 28, 1987) to combine a one-way clutch and torque limiting means in a parallel relationship so that an excessive braking force may be prevented from being transmitted to the rear wheels. In another copending US patent application Serial No. 148,804 filed Feb. 17, 1988 (based on Japanese patent application No. 62-17732 filed on Jan. 28, 1987), there is disclosed a combination of a pair of one-way clutches connected in parallel and opposite relationship with one of the clutches incorporating sprags which are made speeddependent in their function through centrifugal forces acting on them so that the braking force may be prevented from being transmitted to the rear wheels when the vehicle is running at high speed.